Managing the Small Details

Small wind electric systems can have alternating current (AC) outputs, direct current (DC) outputs or both, including electrical energy storage, such as batteries. Article 694 in the 2011 National Electrical Code (NEC) can be a useful reference for small wind (turbine) electric systems that consist of one or more wind electric generators with individual generators having a rated power up to and including 100 kilowatts (kW). These systems can include generators, alternators, inverters, controllers and transformers as well as other pertinent equipment.

Circuit sizing and current—694.12The requirements for designing and installing wind generators are very similar to those for photovoltaic systems.

Designers must consider the maximum current for a circuit and then perform the calculation by applying the requirements in (A)(1) through (A)(3) to 694.12 as outlined below:

• Section 694.12(A)(1) requires the turbine output circuit current to be based on the circuit current of the wind turbine operating at its maximum output power. Most designers consider this to be a value in watts.

• Section 694.12(A)(2), on the other hand, requires that the inverter’s maximum continuous output circuit current be used when determining the circuit size, which is based on such current in amperes (A).

• Section 694.12(A)(3) appears to cover the requirements for a stand-alone inverter’s maximum input current. Such current is considered continuous duty when producing rated power at the lowest input voltage.

Ampacity and overcurrent device ratings—694.12Designers must evaluate and classify the operation of the currents from small wind turbine electrical systems before attempting to size the overcurrent protection devices (OCPDs). The following procedures can be used to calculate, in amperes, the current value necessary to size and select OCPD ratings:

• Section 694.12(B)(1) requires that designers consider the turbine system’s electric currents generated from small wind generators to be rated as continuous. The OCPDs’ sizes can be determined by applying this concept. The definition of “continuous,” per Article 100, is any load that is capable of operating for three hours or more without an interruption.

• Section 694.12(B)(2) governs the appropriate method for calculating the current, in amperes, to size the conductors and OCPDs. The procedure is very clear that these components must be calculated and sized to carry not less than 125 percent of the maximum current as determined by the requirements set forth in 694.12(A), based on a continuous duty of operation. Where the OCPD does not correspond with the ampacity of the conductors, the next size device can be used per 240.4(B), but it can’t be rounded up if it is more than 800A per 240.4(C). The requirements in the Ex. to 694.12(B) are usable when such conditions are applicable.

Proper protection benefits—694.15(A)Some small wind electric systems rely on the turbine output circuit to regulate turbine speed. Inverters may also operate in reverse for turbine startup or speed control. Possible backfeed of current from any supply source (could be ground-fault current), including a supply through an inverter to the wind turbine output circuit, must be considered when determining whether adequate overcurrent protection (OCP) from all sources is provided. Because, of this problem, approximate protective devices must be sized, selected and located at each source to protect the components of the wind generator’s electric system. OCP is not always required when conductors are sized in accordance with 694.12(B) and where the maximum current from all electric sources does not exceed the ampacity of the conductors.

Power transformers—694.15(B)When power transformers are used, the OCP must be provided as outlined in 450.3 by considering one side of the transformer, then the other side of the transformer, as the primary. For the protection scheme, designers and installers can reference Tables 450.3(A) and (B) and apply as necessary. OCP can be eliminated for power transformers that have a current rating on the side that is connected to the inverter output, if such rating is not less than the continuous output current rating of the inverter.

Disconnected means—694.20A switch, circuit breaker or other device, either AC or DC, must not be installed in the grounded conductor if operation of that switch, circuit breaker or other device used leaves the grounded conductor in an ungrounded and energized state. However, such devices must disconnect all ungrounded conductors from the turbine generator’s power supply source.

The exception, when applied, does not require a turbine output circuit disconnecting means.

Additional provisions—694.22For more information on the marking, location and readily or not readily accessibility of small wind generator disconnects, see 694.22(A) through (D).

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.